I read a study that stated that head and arms are insufficient. It seems that a large surface area (back, chest) is preferable. Several studies indicate that 5 - 10 minutes of exposure a day (summer) is sufficient.

There are a number of competing studies and no conensus as to how much D is required per day, so I'm not sure that there is an answer to your question. I can say that Vitamin D is processed in a systemic fashion, wherever it is synthesized in the pigmant of the skin. Keeping one arm bathed in sunlight, and the other in darkness will not deprive the other of the Vitamin D.

From a practical point of view, you want to maximize your D production, and minimize damage from UV correct? This supports marcusl in that a large surface area in a short time is better than your arms baking for hours.